Before the early 1980s, golf balls generally were thread wound golf balls which comprised a thread wound core and a balata cover, which mainly contains transpolyisoprene, covering the core. In the late 1980s, a solid core was developed instead of the thread wound core and was covered with an ionomer resin of ethylene-methacrylic acid copolymer commercially available from DuPont Company, instead of the balata cover to form a two piece golf ball. The two piece construction is being employed at present.
The two piece golf balls have become very popular among most of amateur golfers in these days, while the thread wound golf balls covered by the balata cover are regularly used only by skilled amateur golfers or professional golfers. This would be based on the reason that the ionomer resin has high cut resistance and is cheaply available, in comparison with the balata resin. The ionomer resin is commercially available from DuPont Company as SURLYN and in Japan available from Mitsui DuPont Polychemical Co., Ltd. as HI-MILAN.
The ionomer resin which is used for the cover of golf balls, however, is generally an ethylene-methacrylic acid copolymer, and no golf balls of which the cover is an ethylene-acrylic acid copolymer have been ever made and available. For example, U.S. Pat. No. 3,819,768 and Japanese Kokai Publication 119766/1982 suggests that a golf ball cover is formed by ionomer resins, but all ionomer resins are SURLYN (or HI-MILAN), i.e. the ethylene-methacrylic acid copolymer.
The two piece golf balls of which the cover resin is the ethylene-methacrylic acid copolymer are still desired to improve ball velocity, flying distance and feeling when hitting the balls. Especially, flying distance and ball velocity are desired to be improved, because every golfer wishes to increase flying distance.